Wet air oxidation (WAO) is a well-known technology for treating process streams and is widely used, for example, to destroy pollutants in wastewater. The process involves aqueous phase oxidation of undesirable constituents by an oxidizing agent, generally molecular oxygen from an oxygen-containing gas, at elevated temperatures and pressures. In addition, the process can convert organic contaminants to carbon dioxide, water, and biodegradable short chain organic acids, such as acetic acid. Inorganic constituents including sulfides, mercaptides, and cyanides can also be oxidized. WAO may be used in a wide variety of applications to treat process streams for subsequent discharge, in-process recycle, or as a pre-treatment step for a conventional biological treatment plant.
Systems employing activated carbon, such as powdered activated carbon treatment (PACT) and granular activated carbon (GAC) systems, utilize different activated carbons to remove contaminants from various fluid streams. At some point, the ability of the activated carbon to remove further contaminants decreases. Wet air oxidation (WAO) systems have thus been employed to regenerate spent carbon. When biological solids are also present, the WAO system may also simultaneously oxidize the biological solids accompanying the spent carbon.
One common problem in known WAO systems is the build up of solids within components, such as fluid pathways, heat exchangers, and the like of the system. This “build up” is referred to as scaling. One such scale forming contaminant is calcium sulfate. To prevent the build up of calcium sulfate or like materials in the components of a WAO system, the scale forming contaminant must be periodically removed from the system. A number of techniques have been developed for removing scale forming contaminants to prevent disruption or loss of efficiency of the WAO system. These known solutions, however, have notable deficiencies. For example, one proposed solution utilizes a nitric acid wash through the relevant components. This process, however, is time consuming and can be dangerous as operators are required to handle concentrated and/or hot nitric acid. The acid wash process also requires a WAO system to be shut down, which typically results in a larger system being installed to account for the associated system downtime.
Further, WAO systems are currently built with “double pipe” heat exchangers to accommodate material heading to the reactor of the WAO system, as well as material from the reactor. Such heat exchangers allow the effluent from the WAO system, which is generally very hot, to provide heat to the material to be input into the WAO system. In such a case, when a scale forming contaminant, such as calcium sulfate, is present in the material to be input to the WAO system, the calcium sulfate may collect in undesired locations, such as on the tube side of the heat exchanger portion due to the inverse solubility of calcium sulfate. This scaling problem is very serious and may, in fact, lead to complete system shutdown.